Myotonic Dystrophy 2 (DM2) is a common muscular disease clinically related to Myotonic Dystrophy 1 (DM1). DM2 is caused by expansion of CCTG repeats in intron 1 of the zinc finger factor encoding gene, ZNF9, located on chromosome 3q. While some symptoms in the patients with DM1 and DM2 are similar, others are specific just for one type of disease. DM2 specific features include a lack of congenital form of disease, lack of developmental brain abnormalities, lack of retardation and significantly milder symptoms in skeletal muscle. The main focus of research in my laboratory is the investigation of molecular bases for DM1 and DM2. In the course of our studies on DM1, we found that expansion of CTG triplet repeats in DMPK gene causes DM1 pathology through un-translated RNA CUG repeats. CUG repeat expansion affects two RNA-binding proteins, CUGBP1 and MNBL. DM2 is also caused by expansion of untranslated CCTG repeats suggesting similar RNA-dependent mechanisms for DM1 and DM2. However, specific features of DM2 phenotype suggest that molecular mechanisms of DM2 are not identical to those of DM1. We found that RNA CCUG repeats in DM2 interact with distinct RNA-binding proteins and with large protein-protein complexes, designated as Mega Protein-Protein Complexes (MPPC). These complexes bind specifically to CCUG repeats but not to CUG repeats. MPPC are observed in cytoplasm and in nuclei of DM2 cells. The amounts of cytoplasmic MPPC are increased in DM2 myoblasts, while the levels of nuclear MPPC are reduced in DM2 patients. The major hypothesis of this application is that CCUG repeats cause DM2 pathology via changes of biological functions of large protein-protein complexes. Three Specific Aims are designed to test this hypothesis. We have found that cytoplasmic MPPC contains 9 proteins including CUGBP1, eJF-2a and eIF-2p, all three proteins are involved in the regulation of translation of mRNAs. Specific Aim 1 will identify other protein components of cytoplasmic MPPC and examine the role of cytoplasmic MPPC in mRNA translation. Specific Aim 2 will determine identity of proteins within nuclear MPPC, examine if this complex plays a role in splicing and investigate if the alterations in nuclear MPPC cause abnormal splicing in DM2. Specific Aim 3 will study the mechanisms by which expansion of CCUG repeats increases amounts of MPPC in cytoplasm of DM2 cells and reduces amounts of MPPC in nuclei.